Power conversion refers to the conversion of one form of electrical power to another desired form and voltage, for example converting 115 or 230 volt alternating current (AC) supplied by a utility company to a regulated lower voltage direct current (DC) for electronic devices, referred to as AC-to-DC power conversion, or converting. Power converters are included as part of the chargers and adapters used by electronic devices such as mobile phones, tablets, laptops, and other mobile electronic devices.
As mobile electronics devices continue to grow in popularity, there are increasing demands for miniaturization (high power density) and portability. In order to achieve such miniaturization and portability, higher switching frequency and higher efficiency are demanded. The size of a power converter is generally related to the device switching frequency and efficiency. A higher switching frequency can decrease the size of energy storage components such as electromagnetic components and electrostatic components. Higher efficiency can decrease the size of a heat sink needed to cool the device. As such, high frequency and high efficiency are future trends in the electronics technology.
A switched-mode power supply, switching-mode power supply or SMPS, is a power supply that incorporates a switching regulator. While a linear regulator uses a transistor biased in its active region to specify an output voltage, an SMPS actively switches a transistor between full saturation and full cutoff at a high rate. The resulting rectangular waveform is then passed through a low-pass filter, typically an inductor and capacitor (LC) circuit, to achieve an approximated output voltage. The switch mode power supply uses the high frequency switch, the transistor, with varying duty cycle to maintain the output voltage. The output voltage variations caused by the switching are filtered out by the LC filter.
An SMPS can provide a step-up, step-down or inverted output voltage function. An SMPS converts an input voltage level to another level by storing the input energy temporarily and then releasing the energy to the output at a different voltage. The storage may be in either electromagnetic components, such as inductors and/or transformers, or electrostatic components, such as capacitors. Advantages of the SMPS over the linear power supply include smaller size, better power efficiency, and lower heat generation.
In order to achieve high frequency and high efficiency, different soft switching technologies, topologies, and converters are emerging, such as converters operating at very high frequency (VHF). Such VHF converters can operate at above 10 MHz switching frequencies. For example, E-type converters and Fai 2 converters can achieve full zero voltage switching (ZVS) on/off for all switch components, such as transistor switches and diodes. FIG. 1 illustrates a schematic circuit diagram of a conventional boost Fai 2 converter. FIG. 2 illustrates a schematic circuit diagram of a conventional isolated boost Fai 2 converter. Each of the Fai 2 converters shown in FIGS. 1 and 2 include a Fai 2 inverter and an E-type rectifier. The Fai 2 inverter in the boost Fai 2 converter of FIG. 1 includes an inductor L1, a transistor switch Q1, a capacitor C1, an inductor L2, and a capacitor C2. The Fai 2 inverter in the isolated boost Fai 2 converter of FIG. 2 includes the inductor L1, the transistor switch Q1, the capacitor C1, the inductor L2, the capacitor C2, and a transformer T1. The transformer T1 includes a primary winding P1 and a secondary winding S1. In both the Fai 2 inverters of FIGS. 1 and 2, the inductor L2 and the capacitor C2 are connected in series and resonate at close to twice the frequency of the transistor switch Q1 switching frequency. The E-type rectifier includes an inductor L3, a capacitor C3, a diode D1 and an output capacitor Cout. A load represented as R_load is connected across the output capacitor Cout. The inductor L3 and the capacitor C3 form an LC network to adjust the output power. For example, the output voltage Vout can be expressed by the equation: Vout=(Req*Z2)/(Req*(Z1+Z2)+(Z1*Z2))*Vac, where Z1 is the impedance of the inductor L3, Z2 is the impedance of the capacitor C3, and Req is an equivalent load seen from the anode of diode D1.
FIG. 3 illustrates voltage waveforms corresponding to the isolated boost Fai2 converter of FIG. 2. The waveform 2 shows the voltage stress Vd2 of the rectifier D1. The waveform 4 shows the drain current Ids of the transistor switch Q1. The waveform 6 shows the driving signal Vgs_Q1 applied to the transistor switch Q1. The waveform 8 shows the drain to source voltage Vds_Q1 of the transistor switch Q1. The waveforms 2-8 show the Fai2 converter is a multi-resonant converter due to the multiple resonant circuit components C1, C2, L2. The inductor L2 and the capacitor C2 are connected in series and resonant at close to twice the frequency of the transistor switch Q1 switching frequency. This results in a low impedance value (close to zero) across the drain to source of the transistor switch Q1 at the second harmonic. The inductor L1 and the capacitor C1 are tuned to make the impedance across the drain to source of the transistor switch Q1 at the fundamental harmonic far greater than the impedance across the drain to source of the transistor switch Q1 at the third harmonic. As a result of such impedance characteristics, the voltage Vds_Q1 has a waveform primarily influenced by the fundamental harmonic and the third harmonic, the contribution attributed to the fundamental harmonic being significantly greater than that of the third harmonic. This results in the waveform 8, the voltage Vds_Q1, having a shape similar to a trapezoid. The trapezoidal waveform provides a lower peak voltage, as compared to a non-Fai 2 converter, and also enables improved zero voltage switching (ZVS) of the transistor switch Q1.
For conventional Fai 2 converters, the voltage stress of the switching components is 2 to 3 times the input voltage. The isolated resonant converter, such as the isolated Fai 2 converter of FIG. 2, has the same characteristic, and the voltage stress on the secondary rectifier diode is 3 to 5 times the output voltage. The higher the voltage stress of the secondary rectifier diode, the higher the conduction voltage drop of the secondary rectifier diode, which reduces overall converter efficiency.